1. Field of the Invention
The present invention relates to display devices driven to perform a display operation by a drive signal being applied to a pixel electrode through a switching element and, more particularly, to an active matrix display device of the type having pixel electrodes arranged in a matrix for high density display.
2. Description of the Prior Art
Display devices, such as a liquid crystal display device, an EL display device, and a plasma display device, have been known in which a display pattern is formed on the screen by selectively driving pixel electrodes arranged in a matrix fashion. A voltage is applied between a selected pixel electrode and counter electrode opposed thereto whereby the display medium present therebetween is optically modulated. Such optical modulation is visually observed in the form of a display pattern. A driving system for pixel electrodes of the active matrix drive type has been known such that independent pixel electrodes are arranged in position and a switching element is connected to each of the pixel electrodes for driving same. Switching elements for selectively driving pixel electrode, such as TFT (thin film transistor) elements, MIM (metal-isolator-metal) elements, MOS transistor elements, diodes, and varistors, are generally known. The active matrix drive system exhibits high-contrast display capability, and is already put in actual use for application to liquid-crystal televisions, computer terminal display devices and the like.
When employing such type of display device for high density display operation, it is necessary to arrange a very large number of pixel electrodes and switching elements in position. However, there may be cases in which some switching elements have been already formed as defective elements when they are formed on a substrate. Pixel electrodes connected to such a defective element will produce a pixel defect which is not contributive to the display.
A construction for correcting pixel defects is disclosed in, for example, Japanese Laid-Open Patent Publication No. 61-153619. According to the construction, a plurality of switching elements are provided for each pixel electrode. One of the plurality of switching elements is connected to a pixel electrode, and the others are not connected to pixel electrodes. If the switching element connected to the pixel electrode should go wrong, the switching element is disconnected from the pixel electrode by means of a laser trimmer, an ultrasonic cutter or the like, and another switching element is connected to the pixel electrode. Connection between the switching element and the pixel electrode is effected by depositing fine pieces of a conductor by means of a dispenser or the like, or applying a coat of such material as Au or Al to a specified portion on the substrate. In Japanese Laid-Open Patent Publication Nos. 61-56382 and 59-101693 there is disclosed a construction such that a laser beam is applied to cause metal melting thereby to cause the individual metal layers to be electrically connected.
The correction of defects according to the foregoing constructions must be carried out when the active matrix substrate is in its condition prior to being assembled into a display device. However, it is extremely difficult to locate any pixel defect in the stage of active matrix substrate. Especially in the case of a large-size display device having as many pixels as 100,000 to 500,000 or more, it is necessary to employ high-precision measuring instruments in order to detect electrical characteristics of all the pixel electrodes involved and to locate defective switching elements. This complicates the process of inspection and hampers mass-production economy, thus resulting in increased cost. For this reason, as a matter of fact, it is impractical to carry out such pixcel defect correction of the active matrix substrate before assembling the display device using such means as laser beams with respect to large-type display devices having a large number of pixels.
FIG. 5 shows one example of a conventional active matrix substrate having a redundant arrangement. A gate bus line 21 which functions as a scanning line, and a source bus line 23 which functions as a signal line are provided in intersecting relation on a insulative substrate. A pixel electrode 40 is formed in a region surrounded by the gate bus lines 21 and source bus lines 23. A gate bus branch line 22 extends from a location on the gate bus line 21 adjacent to an intersecting point of the gate bus line 21 and source bus line 23 and toward the region in which the pixel electrode 40 is formed. Formed on the gate bus branch line 22 is a thin film transistor (hereinafter referred as "TFT") 31 as a switching element. A source electrode 61 of the TFT 31 is connected to the source bus line 23, and a drain electrode 72 thereof is connected to the pixel electrode 40. A spare TFT 34 is also formed on the gate bus branch line 22. A source electrode 64 of the spare TFT 34, as is the case with the source electrode 61 of the TFT 31, is connected to the source bus line 23. However, the drain electrode 74 of the spare TFT 34 is not connected to the pixel electrode 40 but is provided in proximity to the pixel electrode 40 so that it may be later connected thereto.
In this active matrix display device, if a pixel defect occurs as a result of some trouble in the TFT 31, the spare TFT 34 is used to correct the defect. The spare TFT 34 is electrically connected to the pixel electrode 40. As stated above, this connection is effected by depositing fine conductor pieces by means of a dispenser or the like, or applying a coat of Au, Al or the like to specified site on the substrate, or melting the metal layers by application of a laser beam to thereby provide electrical connection. As already mentioned, however, these methods of correction must be carried out when the provision of TFT's on the substrate is completed, and cannot be carried out in the assembled display device itself, in which the location of the pixel defect can easily be determined.
In a display device employing an active matrix substrate as shown in FIG. 5, pixel defects caused by an insulation failure or the like of the pixel electrode 40 cannot be corrected. In order to reduce the effect of such pixel defect occurrence, it is conceivable to divide the pixel electrode 40 into a plurality of divisional pixel electrodes and to provide each divisional pixel electrode with a TFT. According to such arrangement, even if any pixel defect should occur as a result of some isolation failure of a divisional pixel electrode, it is possible to prevent the pixel defect from extending to the whole pixel electrode.
In such a display device, while it is possible to avoid the entire pixel electrode suffering a pixel defect as a result of any insulation failure or the like of a divisional pixel electrode, the entire pixel electrode cannot normally function as such when some failure has occurred in one of the TFT's connected to the divisional pixel electrodes. As such, considering the problem of TFT defects only, the substrate shown in FIG. 5 is advantageous only because of its redundancy.